We recently showed that human blood monocytes bind fibrinogen through a genuine surface antigen only in part similar to the platelet GP Ilb/IIIa. Moreover, some anti-GP IIb/IIIa Mabs cross-react with monocytes. In this study we used the 7E3 Mab which preferentially binds to the activated conformation of the platelet GP IIb/IIIa to characterize the dynamic mechanism of "exposure" of the monocyte fibrinogen receptor. 7E3 Mab (25 μg/ml) completely suppressed the binding of i25I-fibrinogen to ADP (10μM)-stimulated monocytes. However, differently from the platelet GP IIb/IIIa, 125I-7E3 binding to unstimulated monocytes was a non-specific and non-saturable reaction. In contrast, after stimulation with ADP (10 μM), suspensions of human monocytes bound 125I-7E3 with saturation of 25-30 μg/ml of added Mab. Scatchard plot analysis was a single-affinity straight line revealing 97,400 binding sites/monocyte with a dissociation constant of 5.2×10−8 M. The monocyte surface antigen uniquely expressed after ADP-activation recognized by 7E3 was visualized by immunoprecipitation studies. Surface iodinated platelet lysate subjected to immunoprecipitation with 7E3 revealed a single band with molecular weight (Mr) of 116,000 corresponding to the platelet GP IIb/IIIa. In contrast, monocytes showed a dimeric surface antigen precipitated by 7E3 in two subunits with Mr=l55,000 and 95,000 respectively. These data indicate that the adhesion properties of the monocyte fibrinogen receptor defined by an anti-platelet GP IIb/IIIa cross-reacting Mab are structurally and functionally distinct from those of the platelet receptor.
Resolution of chronic idiopathic thrombocytopenia purpura following syngeneic peripheral blood progenitor transplant
